thermal paste may help a little i'd guess but if it's a stainless thermowell then it's not the best conductor. given it's a fermenter making a copper one isn't ideal i'm led to believe.

considered it myself and decided to use brewbuilder stainless probes and a cheap pid as the display (i looked for ages for just display screens but they were more exclusive). easily removable for cleaning i desired too..

I have found the same - with my boiler pt100, which is in a thermowell, with thermal grease, I've found that the reading is as much as 7 degrees C behind, while I'm chilling with my copper coil chiller. I have subsequently reasoned that the best methos is to forget thermowells (maybe OK for fermenters) and fix the pt100 (with screw thread) direct into the line of wort.

If you have a 1/2" NPT/BSP threaded pt100, then a female threaded tee is best (you probably already know all this, it works brililantly on my HERMS, and I get very rapid feeback.